Electrotactile vocoder using handset with stimulating electrodes

ABSTRACT

An electrotactile vocoder includes a handset (3) carrying stimulating electrodes (9) positioned adjacent openings (8) in the handset and electrically contacting the fingers when the handset is worn to cause stimulation of the digital nerves of the fingers, a speech processor/stimulator unit (2) for producing electrical stimuli at the electrodes (9) based on incoming speech and other information received by a microphone (1), the stimulator unit including circuit means for applying stimulating currents to the electrodes (9), the speech processor unit including means for encoding the presence of unvoiced speech components or for encoding information to a first formant F1 in addition to information relating to a second formant F2 and for applying the stimulating currents to selected pairs of electrodes.

FIELD OF THE INVENTION

This invention relates to improvements in electrotactile vocoders.

BACKGROUND OF THE INVENTION

Hearing impairment limits access to speech cues and other acousticinformation. The degree and ability of patients to use any residualhearing will determine how much the hearing-impaired person is able tounderstand using hearing aids. Most people with a significant hearingloss will need to use lip-reading to gain further speech information.Some cues to speech, such as place of consonant articulation, arerelatively visible, however other speech cues, such as the formants ofback vowels, and in particular consonant voicing, cannot bedistinguished using visual cues. The role of tactile devices is toprovide supplementary information which the hearing-impaired person canadd to the visual and auditory information they receive. Tactile devicesdo not function as a complete speech information channel, and the wearermust continue to use visual and any available auditory information tomaximise speech understanding.

Tactile devices provide speech information through electrotactile orvibrotactile stimulation of the tactile sense. Electrotactile devicesuse electrodes and low level electric current to stimulate the nervesendings of the tactile sense. Vibrotactile devices use vibrators tostimulate the tactile sense. Two different types of approaches may beemployed to present speech information via tactile devices. The speechprocessing approach involves the tactile device selecting from thespeech input certain aspects of the speech which are considered to beimportant for understanding the message. Alternatively, the bandpassapproach involves filtering the entire speech input into a number ofbands which are defined by frequency boundaries.

Many different body sites have been employed for use with tactiledevices, some of the most common are the forearm, wrist, stomach,sternum and fingers. The transducers are usually held firmly against theskin—using elastic straps or a type of handset. See for example U.S.Pat. No. 4,982,432 (Clark et al) and U.S. Pat. No. 5,035,242 (Franklinet al). In general the user must also wear a microphone and a box whichcontains the speech processor or filter for the speech input, thestimulator circuitry and the power supply.

In our U.S. Pat. No. 4,982,432, in particular, the entire contents ofwhich are incorporated herein by cross-reference, we have disclosed anelectrotactile vocoder which has been found to assist severely andprofoundly deaf adults, as well as providing assistance to speechperception and to articulation when used as a self-feedback device.Information provided through the tactile sense can be used directly, orcombined with information provided through other sense modalities, suchas vision (speech reading) or audition (hearing aids).

Trials of the electrotactile vocoder described in the above UnitedStates patent have been undertaken, and these trials have clearlyestablished that the device can provide assistance to speech perception,and to articulation to persons having hearing impairment.

The electrotactile vocoder described in the above US patent comprisestwo main component elements:

(i) a handset and associated cabling, which incorporate eight electrodespositioned over the digital nerve bundles on each side of the fourfingers of one hand, the purpose of which is to transmit theelectrically-encoded speech information to the user; and

(ii) a speech processor/stimulator unit and associated input microphoneand cabling; the purpose of which is to detect incoming speechinformation, analyse and process this information, extract certainspecific speech feature components as directed by the speech processingprogram implemented, electrically encode this information as changes inthe stimulus electrical parameters of electrode selected, stimulus pulsewidth, and stimulus pulse rate, and send the electrical stimulus to theelectrode handset for delivery to the user.

The trials referred to above have established that the electrotactilevocoder described in the above United States patent needed improvementin the following areas:

(i) the design of the electrotactile handset through which informationis transmitted to the user and particularly the use of a large returnelectrode at the wrist;

(ii) the design of the speech processing strategy which is used toencode speech information for the user;

(iii) the circuitry and method of providing the electrical stimulus tothe user, including the need for a large return electrode at the wrist,and for programming of the speech processor.

SUMMARY OF THE INVENTION AND OBJECT

It is the object of the present invention to provide improvements inelectrotactile vocoders of the type described in the above US patent tothereby improve the ability of the electrotactile vocoder to providebenefit to hearing-impaired persons.

In a first aspect, the invention provides an electrotactile vocoderincluding a handset carrying stimulating electrodes which are positionedby the handset when worn to be in electrical contact with the fingers tofacilitate stimulation of the digital nerves of the user, a speechprocessor/stimulator unit for producing electrical stimulus at theelectrodes based on incoming speech and other information, saidstimulator including circuit means for applying stimulating current tosaid electrodes and for switching at least selected ones of saidelectrodes to a non-active state when stimulating current is applied toany one of the other electrodes, and means for utilising the electrodesin said non-active state as ground or return electrodes.

By adopting the above strategy, the wrist electrode described in theUnited States patent referred to above is no longer required therebysubstantially increasing the ease of use of the device, including theability to provide an essentially one-piece handset which may beconveniently used by the hearing-impaired person and overcoming theelectrophysiological problem of dielectric breakdown at the wristelectrode, which causes unpleasant levels of stimulation at the largereturn electrode under specified conditions, including dry climates.

In a preferred form, all electrodes which are in said non-active stateare utilised as ground or return electrodes.

In another aspect, the invention provides an electrode handset for anelectrotactile vocoder, including a body, four finger members extendingoutwardly from said body, each finger member having a shaped opening forreceiving a finger and defining a ring-like member encircling the fingerwhile leaving the fingers substantially free, opposed pairs ofelectrodes positioned adjacent each opening so as to make electricalcontact with opposite sides of each finger in the region of the digitalnerve when the fingers are inserted into the finger openings of thefinger members, said ring-like members holding said electrodes inelectrical contact with said fingers while allowing flexing of the handor fingers without substantially affecting said electrical contact, saidbody also including an opening defining a wrist engaging strap forholding the handset on a hand with said electrodes engaging saidfingers.

The body is most conveniently made of a resilient material, such asneoprene, and conductive wires extend from the electrodes to anelectrical connector adapted for connection to the speechprocessor/stimulator of an electrotactile vocoder.

The wires are preferably mounted on the surface of the electrode handsetbody and may be retained in position by an adhesive overlay or any othersuitable means. In this way, the wiring is integrated into the body ofthe handset thereby reducing the likelihood of dielectric breakdownwhile ensuring a pleasant and effective stimulus delivered through theelectrodes.

The speech processor/stimulator unit also incorporates a speechprocessing strategy which modifies the strategy utilised in the case ofthe electrotactile vocoder described in the above US Patent. In thatstrategy which was based on the multichannel cochlear implant developedby The University of Melbourne and Cochlear Limited and availablecommercially from Cochlear Limited, specific speech features (secondformant frequency, second formant amplitude, and fundamental frequency)were electrically encoded and provided to the user through a specificpattern of stimulation of single electrodes. Following significantpsychophysical and speech research, a new strategy, designated the UF2strategy, has been developed which incorporates several novel featuresnot anticipated from the earlier research and testing nor evident inpublished literature on tactile psychophysics. The specific encoding ofan “unvoiced” stimulus is a novel approach which has not been employedin other devices. While other tactile devices have presented signals tothe voiced/unvoiced context, they have all approached this problem byproviding a signal to a “voiced” consonant. This approach has beenineffective, since both vowels and some consonants are voiced, and usershave in general been unable to detect the difference between anunvoiced/voiced consonant at the start of a word immediately followed bya voiced vowel.

Thus, in another aspect, the invention provides an electrotactilevocoder including a handset carrying stimulating electrodes which arepositioned by the handset when worn to be in electrical contact with thefingers to facilitate stimulation of the digital nerves of the user, aspeech processor/stimulator unit for producing electrical stimulus atthe electrodes based on incoming speech and other information, saidstimulator unit including circuit means for applying stimulatingcurrents to said electrodes, said stimulator unit including means forencoding the presence of unvoiced speech components, or for encodinginformation relating to a first formant F₁, in addition to informationrelating to a second formant F₂, and for applying stimulating currentsto selected pairs of electrodes.

Thus, the new speech processing strategy includes the use of a “paired”rather than “single” dimension (ie. multiple-electrode sequentialstimulation) to provide additional information. The use of thisdimension was based on novel experimentation showing that users could infact recognise paired stimulation accurately. Analysis of results of useof the paired/single dimension was found in psychophysical testing tosignificantly increase the information transmission capabilities of thedevice.

In a preferred form, the speech processor/stimulator unit includesfilter means for enabling a stimulating current to represent highfrequency components of the speech signal.

The stimulating currents are preferably applied to the first or secondelectrodes and one of the third to eighth electrodes when an unvoicedstimulus is required.

The novel modifications to the Speech Processor Strategy include thefollowing:

incorporation of specific filters to provide a specific stimulus inrelation to high frequency components in the speech signal;

use of a paired stimulus rather than a single stimulus to encode thepresence of an unvoiced versus voiced speech component, or to encode F1information in addition to F2;

implementation of this strategy to incorporate a paired stimulus beingdelivered to electrode 1 or 2, plus one of electrodes 3 through 8 inrelation to an unvoiced stimulus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the component parts of the electrotactile vocoder in use;

FIG. 2 is an exploded view of the handset for the electrotactile vocoderof FIG. 1;

FIG. 3 is a top plan view of the handset showing the specialconfiguration of the finger holes or rings;

FIG. 4 is a bottom plan view of the handset with the flap raised toexpose the electrical connector;

FIGS. 5 and 6 illustrate the fitting of the handset to the hand of theuser; and

FIGS. 7 to 12 illustrate typical stimulator circuitry for the speechprocessor 2, in which: FIG. 7 shows the stimulator power supply, FIG. 8shows the Stimulator logic circuit, FIG. 9 illustrates a typicalstimulator output circuit for each electrode, and FIGS. 10 to 12 showthe interface circuitry for the stimulator.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows the three main parts of the electrotactile vocoderembodying the invention: a microphone 1, a speech processor 2 and ahandset 3.

The microphone 1 plugs into a socket 4 on top of the speech processor 2and the speech processor has two controls, an on/off switch 5 and asensitivity control 6. By turning up the sensitivity control it ispossible to make the system more sensitive. When set on a highersensitivity the speech processor 2 will pick up softer speech andenvironmental sounds around the user. The sensitivity control is not avolume control. The levels programmed into the speech processor duringmapping act as the volume control for the system.

As shown in FIG. 2, the flexible handset 3 is die cut from Neoprene orlike resilient material and has a wrist strap 7 and four shaped rings oropenings 8 for each of the four fingers of the hand. An electrode 9 ispositioned on either side of each ring 8 to provide eight fingerelectrodes altogether. The electrodes 9 are electrically connected bywires 10 to a connector 11, the wires 10 and electrodes 9 being held inplace on the handset 3 by an overlying layer 12 of polypropylene, suchas Melco, adhesively secured to the handset 3 by a hot melt adhesive.The layer 12 includes a cover 13 which protects the connector 11 asshown in FIG. 3.

The rings 8 are defined by shaped openings in the handset 3 with eachshaped opening including lobes 14 providing support for the electrodes9. The lobes 14 are positioned at either side of the rings 8 toresiliently hold the electrodes 9 in contact with the sides of eachfinger when the handset 3 is fitted to the hand of a user.

To put the handset on:

(i) plug the small end of the handset cable into the row of pins underthe flap on the underside of the handset;

(ii) ease the cable connector into the square socket until it lies flatand close the flap and seal using the velcro dots;

(iii) place the handset on the table with the electrodes facing up andthe fingers pointing towards you (refer to FIG. 4);

(iv) place the tips of the fingers of your left hand into the rings;

(v) lift the handset up by the wrist strap and gently pull the handsetonto the fingers;

(vi) pull the wrist strap forward over the finger tips and put your handthrough the wrist strap (refer to FIG. 5) and pull the wrist strap backaround your wrist.

To take the handset off:

(i) pull the wrist strap forward over the fingers and across the back ofyour hand;

(ii) gently ease each ring part way down the fingers before removingthem totally.

The handset cable 10 has a small connector 11 on one end, which plugsinto the row of pins 12 under the flap on the underside of the handset(refer FIG. 3). The larger connector 13 at the other end of the cableplugs into the second socket 14 on top of the speech processor.

The Speech Processor Unit consists of four main functional parts:

a wide-band directional microphone and associating cabling,

a speech processor circuit,

a stimulator circuit, and

an interface for programming of the speech processor/stimulator.

These parts will be further described in relation to the preferredembodiment.

(i) Input Microphone: The input microphone used is a directional,wide-band microphone, which allows input up to 7000Hz;

(ii) Speech Processor Circuit: The speech processor receives informationfrom the wide-band microphone. In the speech processor circuitry,specific features of the incoming waveform are detected, and measured,according to software directions provided to the circuitry through thespeech processing strategy previously programmed. In the speechprocessor circuit, estimates of a number of speech features can beextracted. The outputs from the speech processor circuitry are thenelectrically encoded as a pulse width modulated signal according to thesoftware directions contained within the speech processor encoderprogram, and the user's individual speech processor map (software whichsets the threshold and comfortable stimulus pulse widths for eachindividual electrode). The speech processor circuit preferablyincorporates a MSP speech processing board from Cochlear Limited. Inaddition, an alternative Digital Speech Processor Board version (using aMotorola 56000 processing chip) can be implemented.

The electrically coded speech information sent to the handset is basedon those features of speech selected by the speech processor. Using thecurrent UF2 speech processing strategy, the speech processor selectsfrom the speech:

1. an estimate of the fundamental frequency (F0) of the speech;

2. an estimate of the second formant frequency (F2) of the speech;

3. the amplitude of the speech; and

4. an indication of the presence of high frequency information above 4ktHz.

5. an indication of the unvoiced/voiced speech feature for consonants.

These speech features are encoded differently by the speech processor(refer to Table 1). As a result of changes in the electrically encodedsignal sent to the handset, changes in these features are perceived bythe user as changes in the stimulation (refer to Table 1). F0 iselectrically encoded as pulse rate. Changes in F0 are perceived by theuser as changes in the “roughness” of the stimulation. F2 iselectrically encoded as the electrode selected to be turned on, and isperceived by the user as the particular electrode providing stimulation.The amplitude of speech is encoded as pulse width. Changes in the pulsewidth of the stimulation is perceived by the user as changes in theintensity of stimulation. The presence of high frequency information,such as is produced by high frequency fricatives, is encoded to bepresented to the user as stimulation on electrode 8.

The presence of an unvoiced consonant is encoded by activation ofelectrode 1, in addition to the F2 or high frequency electrode, andperceived by the user as a “paired stimulus”.

Each speech feature presented is designed to help the user improve theirunderstanding of speech. The perception of roughness (F0) providesinformation on syllable stress and consonant voicing. Use of thisinformation would help the user to perceive syllable number and stress,vowel duration and consonant manner. The perception of which electrodeis stimulating (F2) provides information on vowel formants and highfrequency fricatives. This will enable the user to discriminate F2 anddetect high frequency information.

TABLE 1 Summary of the speech features selected, their electricalcoding, tactile signal and help in understanding speech. ElectricalTactile Speech Feature Coding Parameter Information F0 pulse rateroughness of syllable number & stimulation stress vowel durationconsonant manner F2 electrode electrode provid- vowel formant turned oning stimulation speech waveform pulse width intensity of intonationpattern amplitude stimulation of speech phoneme duration high frequencyelectrode 8 stimulation on presence of high information electrode 8frequency fricative voiced/unvoiced electrode 1 stimulation on presenceof an consonant electrode 1 and unvoiced consonant F2 electrode

Using the software it is possible to implement alternative speechprocessing strategies in the speech processor. This inbuilt flexibilityallows for future implementation of speech processing strategiestailored to provide specific information.

Referring to FIGS. 7 to 11, the stimulator circuitry decodes the pulsewidth modulated (pwm) signal from the processor and houses the circuitryto stimulate the relevant electrode. Novel features incorporated intothe stimulator circuitry include:

the method of implementing the stimulus without use of a separate groundor return electrode, and

the method of electrically isolating the user from the electricalsupply, ensuring patient safety.

Specific details of the circuitry allowing for electrical isolationinclude the following:

Transformer L1 in FIG. 7, typically a Siemens EFD15 No 7 core, suppliesisolated 5V for components and isolated 110V for the stimulation of theelectrode.

Capacitors C4 and C5, in FIG. 7, are used to generate the 110V for thestimulation.

The pulse width modulated signal from the processor is passed throughthe optocoupler, U3 (FIG. 8). This provides isolation between theprocessor and the stimulator.

U4 provides timing for clock used to shift data into the programmablechip U5 (FIG. 8).

U5 is a programmable chip, which decodes the MSP Dampl(Data) signal intothe relevant electrode. FIG. 8 illustrates the circuitry that areimplemented in U5. The circuitry consists of a shift register whichshifts the pwm signal into the binary to decimal decoder. The output ofthe decoder signifies which electrode is selected. The output of U5 issuch that when one electrode is active all the other electrodes areinactive and the inactive electrodes operate as a common/ground. In thisway, no wrist electrode, or other large common/ground electrode, isrequired as in the prior art.

A time delay in the dampl signal is implemented by switching off the110V isolated supply in the 110 return path. There is no current flowingduring the 100us gap. The open signal from U5 controls the switching. Q4in diagram 2 is OFF when the open signal goes HIGH.

At the output, the two MOSFETs form a bipolar current regulator circuit.The capacitor in series with the 1M resistor prevents DC current fromflowing.

separate returns are used for the battery power source B₁, B₂ the MSP 5Vsupply and the opto-coupler U3 (FIG. 8).

FIG. 8 illustrates a switching circuit 010,020,030 which connects theMSP to the common/ground of the inactive electrodes via capacitor C10 toreduce noise.

The interface is powered by two 9V alkaline batteries. These batteriesalso supply the power during programming. Power is not drawn from thecomputer.

The aim of mapping is to ensure that the device user always receives astimulus that is above threshold and pleasant, yet strong enough toconvey the maximum amount of speech information. The parameters used tospecify the electrotactile stimulus are the threshold level (T-level)and comfortable level (C-level) pulse widths. The pulse width may beselected from the range 10 -1000 microseconds. A change in the pulsewidth of stimulation results in a change in the current being used toproduce the stimulus. For the user this results in a change in theperceived strength of the stimulation. Sensitivity to the electrotactilestimulus varies between people and also between the fingers of oneperson. As such, it is important that the parameters of theelectrotactile stimulation can be specified for each of the individualelectrodes.

The T-level will be set at the point at which the wearer is first ableto detect the tactile stimulus. The C-level will be set at the point atwhich the wearer reports the stimulus to be strong but comfortable toreceive ongoing for 5 to 10 minutes. The T and C-levels will vary acrosselectrodes and, even more so, between subjects. In particular, thelevels on the outer edges of the first and fourth fingers have beenfound to be higher than on the other 6 electrodes (perhaps due to thethickness of the epithelium). After all of the levels are set it isimportant for the clinician to sweep across the eight electrodes toallow the wearer to balance the stimulus received on each electrode. Thelevels should be balanced so that when speech information is presentedstimulation is balanced across the electrodes. Otherwise importantinformation, such as loudness (perceived as strength of stimulation) maynot be presented correctly, or weak stimulation may be masked bystimulation on adjacent electrodes.

It is necessary to set a threshold (T-level) and comfortable (C-level)for each electrode for each user. This can be quite an extensive processfor the first map as the appropriate levels and the growth function ofthe sensitivity will vary greatly between different people and betweenthe fingers of the same person. Subjects will not only need to learn themapping process but become accustomed to the character of thestimulation. For this reason the clinician must be particularly carefulwhen setting levels for the first map. Once the likely area of a levelis identified, the mapping process is significantly simplified.

Most users will not find mapping an easy task during the first, andpossibly subsequent sessions. The clinician must be careful to explaineach step—particularly those requiring feedback from the user. As thefeedback required is subjective, it is important that the clinicianlistens carefully and learns to make a judgement regarding the criteriathe person is using to provide feedback. As the user has more experienceregarding the concept of mapping the sensation provided by the devicethey will be more able to make accurate decisions regarding the level ofstimulation. It is suitable during the first mapping session toencourage the user to report on the level of the sensation they arereceiving without considering dynamic range or the actualappropriateness of the comfortable levels selected.

After new T and C-levels are set and balanced they need to be programmedinto the speech processor. This is achieved by creating a “map”. When amap is created it will use the most recent levels entered into theMeasure Thresholds screen. Whenever the T or C-levels are changed a newmap must be created. After the map is created it must be tested by theuser. The act of testing a map writes that map into the memory of thespeech processor. Any map that is written into the speech processor willremain in the processor until the next map is tested ie. written overthe previous map. If testing indicated that the map was appropriate itcan be saved on the user's disk for future use.

The frequency with which particular device users will require re-mappingwill vary. It is important for mapping to occur weekly during the firsttwo months, as levels will stabilise as the user becomes more familiarwith the task and the stimulus. Regular checks will be required afterthis time, and should also be provided following any complaints by theuser about the strength or quality of the stimulation.

PROGRAMMING AND PROGRAMMING INTERFACE

The programming interface is shown in FIGS. 11 and 12. It functions toallow commands from the computer software program to be passed to theSpeech Processor Unit, allowing programming of encoder strategy andpatient map. The Programming Interface has a dedicated power switch.Power is supplied for approximately 1 second when turned on via thesoftware. The power will continue to be on if the connections arecorrect. If the connections between the speech processor and thecomputer are not complete the power will be switched off. This is toensure that there will not be power when the programming system is notconnected correctly or has come loose during use.

The interface system acts as the interface between the computer and thespeech processor. The interface system consists of:

1. the computer cable

2. the programming interface

3. the speech processor cable

The computer cable connects the computer to the programming interface.One end of the computer plugs into the parallel port of the computer(the port usually reserved for the printer), the other end plugs intothe computer port on the programming interface. The speech processorcable plugs into the speech processor port on the programming interfaceand connects it to the speech processor.

The programming interface uses two PP3 9 volt batteries. These batteriesalso supply power to the speech processor whilst it is connected to theinterface. The programming interface must be switched off when not inuse to avoid draining the batteries. When the battery voltage is below 7volts the programming interface may not reliably maintain the connectionto the speech processor. At this voltage level the red “Low Voltage”light on the front panel of the programming interface will illuminate.This light will also be illuminated if the programming interface isconnected to the computer but not turned on.

What is claimed is:
 1. An electrode handset for an electrotactilevocoder, including a body, four finger members extending outwardinglyfrom said body, each finger member having a shaped opening for receivinga finger and defining a ring-like member encircling the finger whileleaving the fingers substantially free, opposed pairs of electrodespositioned adjacent each opening so as to make electrical contact withopposite sides of each finger in the region of the digital nerve whenthe fingers are inserted into the finger openings of the finger members,said ring-like members holding said electrodes in electrical contactwith said fingers while allowing flexing of the hand or fingers withoutsubstantially affecting said electrical contact, said body alsoincluding an opening defining a wrist engaging strap for holding thehandset on a hand with said electrodes engaging said fingers.
 2. Thehandset of claim 1, wherein the body is made from a single piece ofresilient material having an outlined shape defining said fingermembers, said shaped openings being cut from said finger members.
 3. Thehandset of claim 1, wherein said each shaped opening has lobespositioned at either side of each shaped opening, said electrodes beingpositioned on said lobes so that the electrodes are presented and heldin contact with the sides of the fingers in the region of the digitalnerves when the fingers are inserted through the openings and thehandset fully engages the hand.
 4. An electrotactile vocoder including ahandset carrying stimulating electrodes which are positioned by thehandset when worn to be in electrical contact with the fingers tofacilitate stimulation of the digital nerves of the user, a speechprocessor/stimulator unit for producing electrical stimulus at theelectrodes based on incoming speech and other information, saidstimulator unit including circuit means for applying stimulating currentto said electrodes and for switching at least selected ones of saidelectrodes to a non-active state when stimulating current is applied toany one of the other electrodes, and means for utilising the electrodesin said non-active state as ground or return electrodes for said circuitmeans.
 5. The vocoder of claim 4, wherein said circuit means includesmeans for sequentially applying stimulating currents to more than oneelectrode simultaneously.
 6. The vocoder of claim 5, wherein saidstimulating currents are applied to selected pairs of electrodes.
 7. Thevocoder of claim 4, wherein said circuit means includes programmablemeans which applies said stimulating current to selected electrode(s),said circuit means further including a shift register means for shiftinga pulse width modulated signal into a binary to decimal decoder havingan output which causes the circuit means to select the electrode(s), theoutput of said programmable means causing the selected electrode(s) tobe active and all remaining electrodes to be inactive, said inactiveelectrodes acting as common or ground electrodes in the circuit means.8. The vocoder of claim 4, wherein said circuit means further includesmeans to supply isolated voltage to said circuit means for stimulationof said electrode(s).
 9. The vocoder of claim 8, wherein said means tosupply isolated voltage includes optocoupler means for isolating thespeech processor from the stimulator circuit means.
 10. The vocoder ofclaim 8, wherein said means to supply isolated voltage includes meansfor switching the isolated voltage supply off during a time delay periodin the stimulus current, and bipolar current regulator circuit means forpreventing direct current flow to said electrode(s).
 11. Anelectrotactile vocoder including a handset carrying stimulatingelectrodes which are positioned by the handset when worn to be inelectrical contact with the fingers to facilitate stimulation of thedigital nerves of the user, a speech processor/stimulator unit forproducing electrical stimulus at the electrodes based on incoming speechand other information, said stimulator unit including circuit means forapplying stimulating currents to said electrodes, said stimulator unitincluding means for encoding the presence of unvoiced speech components,or for encoding information relating to a first formant F₁ in additionto information relating to a second formant F₂, and for applyingstimulating currents to selected electrodes.
 12. The vocoder of claim 11wherein said stimulating currents are applied sequentially to selectedpairs of electrodes.
 13. The vocoder of claim 11, wherein the speechprocessor/stimulator unit includes filter means for enabling astimulating current to represent high frequency components of the speechsignal.
 14. The vocoder of claim 11, wherein said stimulating currentsare applied to first or second electrodes and one of the third to eighthelectrodes when an unvoiced stimulus is required.